The present invention relates to an apparatus for measuring voltage, and more particularly to an apparatus for measuring voltage of a cell while scanning a group of cells in a cell stack such as a fuel cell stack in which a plurality of cells is electrically connected in series.
A fuel cell is a type of battery which generates electromotive force by electrochemical reaction between hydrogen contained in a fuel gas as a main substance and oxygen. As a cell voltage of the fuel cell only reaches as high as 1 volt, some hundreds of cells are in general electrically connected in series to form a fuel cell stack, which provides a high voltage, 260 volts, for example. If power generation is continued while one cell generates an extremely low voltage, 0.5 volts for example, it may result in damage of the fuel cell stack due to a drop in its aggregate output voltage, which is caused by corrosion occurring in the failed cell. It is typical that measurement and monitoring of a voltage of cell is carried out during operation of a fuel cell stack by scanning cells one by one or a group of cells group by group. In this way, continuous measurement and monitoring of a cell voltage provides prompt notification of occurrence of an abnormal cell, because when a cell or a plurality of cells suffers damage, there is a remarkable drop in voltage. Accordingly, it is possible to immediately stop operation of the fuel cell stack so as to prevent development of damage which results from delay in sensing an occurrence of abnormal operation.
An example of measurement of voltage for an individual cell is disclosed in patent document 1, which uses a terminal shaped like a projection provided for a separator between successive cells. Because a hole is no more required of an end surface of the separator by introduction of the terminal, a socket of a lead wire can be connected with the terminal even if the separator gets thinner. Accordingly, it is possible to measure a voltage for the individual cell even if a fuel cell decreases in dimension.
Various techniques associated with measurement of cell voltage for a fuel cell have been reported. FIG. 5 is a diagram showing a conventional apparatus for measuring voltage. As shown in FIG. 5, a voltage of an individual cell is measured and monitored in the following manner. Cells contained in a fuel cell are divided into a plurality of groups. For example, when switches S31, S32, S33 and S34 belonging to a first group are simultaneously turned on, individual voltages of cells C31, C32 and C33 belonging to the first group are detected by differential amplifiers D31, D32 and D33 of a detecting circuit 30. The voltages are sent to an A/D converter of a central processing unit (CPU) (not shown), where measurement and monitoring are carried out. Subsequently, when the switches S31, S32, S33 and S34 of the first group are simultaneously turned off and switches S35, S36, S37 and S38 belonging to a second group are simultaneously turned on, individual voltages of cells C34, C35 and C36 belonging to the second group are detected by the differential amplifiers D31, D32 and D33, and measured and monitored by the CPU. In this way, it is possible to measure a voltage of each cell in the fuel cell by scanning groups one by one while switches of the groups are turned on and off one after another.
FIG. 6 is a diagram illustrating a polarity of measured voltage for each cell in the detecting circuit shown in FIG. 5. When groups of cells are scanned group by group as described above, voltages imposed on the differential amplifiers D31, D32 and D33 have a constant polarity irrespective of groups. This will allow use of a single power supply, which for example is a one-way power supply having a ground and a positive power source, for each of the differential amplifiers D31, D32 and D33. In this way, the differential amplifiers D31, D32 and D33 can be simply configured. In this connection, filters F31, F32 and F33 disposed in the detecting circuit 30 are elements for removing noise. Buffers B31, B32 and B33 are elements for shaping a wave form of detected voltage.
A technique related to an apparatus for measuring voltage using a flying capacitor is disclosed in patent document 2. The technique is applied to a fuel cell stack in which many cells are electrically connected in series. A capacitor is connected in parallel with each of the cells, and a group is arranged so as to include five cells, for example. Measurement of voltage is carried out by detecting a voltage imposed on a capacitor corresponding to a cell while a switch corresponding to this cell is turned on. By repeating this measurement with switching, it is possible to measure voltages for all the cells of the group. The measurement is carried out for groups one after another so as to complete measurement for all the cells in the fuel cell stack. This technique results in a simplified configuration of circuit. In addition, a technique is disclosed in patent document 3, which measures voltages of individual cells one by one using a plurality of switches in a fuel cell stack, in which a plurality of cells is electrically connected to each other in series. This technique brings about measurement of a voltage of an individual cell with high precision, which does not require a complicated setup.    Patent document 1: Japanese Published Patent Application 11-339828    Patent document 2: Japanese Published Patent Application 2002-156392 (paragraphs 0051-0058, and FIG. 1)    Patent document 3: Japanese Published Patent Application 11-237455 (paragraphs 0018- 0024, FIG. 1 and FIG. 2)
Because a large number of cells are electrically connected to each other in series, the detecting circuit shown in FIG. 5 requires more pieces of switches than number of cells so as to measure a voltage for an individual cell. Specifically speaking, a cell lying in an interface between two successive groups requires two pieces of switches so as to turn off one signal line of a previous group and to turn on the other line of a next group. For example, the cell C34 lying in one interface between a first group and a second group requires the switches S34 and S35. Similarly, the cell C36 lying in the other interface of the second group and a third group requires the switches S38 and S39. It is deduced that switches more than 181 pieces, namely 181+180÷3−1=240 pieces, are necessary in case of a fuel cell stack having 180 cells electrically connected to each other in series, when a group is arranged so as to include three cells. Because PhotoMOS relays having high withstand voltage are in general selected for these switches, the more the number of switches increases, the more expensive material cost will be. In this connection, it may be possible to anticipate some decrease in total number of switches if number of cells belonging to a group is increased. The reason for this is that number of switches decreases, which lie in an interface between successive groups. However, an increase in the number of cells belonging to a group induces an increase in number of filters, buffers and differential amplifiers in a detecting circuit 30, resulting in an increase in material cost of an apparatus for measuring voltage as a whole.
Although the apparatus for detecting voltage disclosed in patent document 2 simplifies a setup of a circuit made of switches, it additionally requires a capacitor on which a voltage of a cell is imposed, resulting in a cost increase. In addition, the circuit for detecting voltage for a cell stack disclosed in patent document 3 has a drawback that a setup of circuit for switching devices turns complex.